f Great Britain Distribution QRA Methodology Updates in CONIFER

The natural gas distribution system has operated safely across Great Britain for around 50 years. It is an established means of supplying fuel for heating and cooking to residential dwellings and non-domestic buildings, and is also used in some industrial processes. In order to meet environmental targets set by the UK Government, multiple gas industry projects have been completed, or are in progress, to investigate the conversion of the existing natural gas system to carry hydrogen. This would make use of a valuable national asset and would help to meet carbon emissions targets.
As part of this programme of work across the gas industry, DNV has carried out a Quantitative Risk Assessment (QRA) of the gas distribution system across Great Britain. Note that the ‘distribution system’ in this report covers a wider scope than the common meaning of the ‘distribution network’. Here the system includes the gas distribution network upstream of the Emergency Control Valve (ECV) and gas use within customer properties downstream of the ECV. The primary aim of the assessment is to quantify the risks posed by a repurposed hydrogen system, and the benefits of risk mitigation measures, where necessary. To put these results in context, the risk associated with the equivalent natural gas distribution system is also quantified.
This work has been carried out in partnership with Cadent, Northern Gas Networks and SGN. Additional information has been supplied by Wales and West Utilities and several of the Independent Gas Transporters, thereby covering almost all the Great Britain distribution system.
This report contains the details of methodology reviews and modelling updates that were carried out as part of the Great Britain distribution system QRA. A separate report details the risk predictions. The updates include some made in response to review by the Evidence Review Group within the HSE, and some that take make use of outputs from other gas industry projects. Major updates made as part of this work include:
• Introduction of non-domestic buildings and Multi Occupancy Buildings (MOBs) into the risk predictions. This report is primarily concerned with defining their characteristics and modifying the risk assessment models to represent their features that are not found in houses.
• Updates to the failure frequencies applied to all parts of the system, from governor kiosks through the distribution network to end use.
• Modifications to the model that predicts gas movement through the ground following a leak from a main or service.
• Changes to the methodology used to predict gas dispersion, movement and accumulation within a building.
• Updates to the treatment of gas explosions inside buildings.
• Several other minor changes and investigations into the sensitivity of various inputs.
This report also contains the risk calculation methodology for governors and benchmarking of the risk predictions against natural gas operational experience.
This report was submitted to HSE for their assessment of the safety evidence for 100% hydrogen heating, which can be found at Hydrogen heating: HSE assessment of the safety evidence - GOV.UK.
Queries should be directed to DESNZ: https://www.gov.uk/guidance/contact-desnz.